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New Geodetic and Gravimetric Maps to Infer Geodynamics of Antarctica with Insights on Victoria Land

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New Geodetic and Gravimetric Maps to Infer Geodynamics of Antarctica with Insights on Victoria Land remote sensing Article New Geodetic and Gravimetric Maps to Infer Geodynamics of Antarctica with Insights on Victoria Land Antonio Zanutta 1,*, Monia Negusini 2, Luca Vittuari 1 , Leonardo Martelli 1, Paola Cianfarra 3 , Francesco Salvini 3, Francesco Mancini 4 , Paolo Sterzai 5, Marco Dubbini 6 and Alessandro Capra 4 1 Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali—Università di Bologna, 40136 Bologna, Italy; [email protected] (L.V.); [email protected] (L.M.) 2 Istituto di Radioastronomia—Istituto Nazionale di Astrofisica, 40129 Bologna, Italy; [email protected] 3 Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, 00146 Roma, Italy; [email protected] (P.C.); [email protected] (F.S.) 4 Dipartimento di Ingegneria Enzo Ferrari—Università degli Studi di Modena e Reggio Emilia, 41125 Modena, Italy; [email protected] (F.M.); [email protected] (A.C.) 5 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale—OGS—“Infrastructures” Section (IRI), 34010 Sgonico (Trieste), Italy; [email protected] 6 Dipartimento di Storia Culture Civiltà—Università di Bologna, 40124 Bologna, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-051-2093111 Received: 30 July 2018; Accepted: 6 October 2018; Published: 10 October 2018 Abstract: In order to make inferences on the geodynamics of Antarctica, geodetic and gravimetric maps derived from past and new observations can be used. This paper provides new insights into the geodynamics of Antarctica by integrating data at regional and continental scales. In particular, signatures of geodynamic activity at a regional extent have been investigated in Victoria Land (VL, Antarctica) by means of Global Navigation Satellite System (GNSS) permanent station observations, data from the VLNDEF (Victoria Land Network for Deformation control) discontinuous network, and gravity station measurements. At the continental scale, episodic GNSS observations on VLNDEF sites collected for 20 years, together with continuous data from the International GNSS Service (IGS) and Polar Earth Observing Network (POLENET) sites, were processed, and the Euler pole position assessed with the angular velocity of the Antarctic plate. Both the Bouguer and the free-air gravity anomaly maps were obtained by integrating the available open-access geophysics dataset, and a compilation of 180 gravity measurements collected in the VL within the Italian National Program for Antarctic Research (PNRA) activities. As a result, new evidence has been detected at regional and continental scale. The main absolute motion of VL is towards SE (Ve 9.9 ± 0.26 mm/yr, Vn −11.9 ± 0.27 mm/yr) with a pattern similar to the transforms of the Tasman and Balleny fracture zones produced as consequence of Southern Ocean spreading. Residual velocities of the GNSS stations located in VL confirm the active role of the two main tectonic lineaments of the region, the Rennick–Aviator and the Lillie–Tucker faults with right-lateral sense of shear. The resulting VL gravity anomalies show a NW region characterized by small sized Bouguer anomaly with high uplift rates associated and a SE region with low values of Bouguer anomaly and general subsidence phenomena. The East and West Antarctica are characterized by a different thickness of the Earth’s crust, and the relative velocities obtained by the observed GNSS data confirm that movements between the two regions are negligible. In East Antarctica, the roots of the main subglacial highlands, Gamburtsev Mts and Dronning Maud Land, are present. The Northern Victoria Land (NVL) is characterized by more scattered anomalies. These confirm the differences between the Glacial Isostatic Adjustment (GIA) modeled and observed uplift rates that could be related to deep-seated, regional scale structures. Remote Sens. 2018, 10, 1608; doi:10.3390/rs10101608 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 1608 2 of 27 Keywords: VLNDEF; gravity anomaly; GNSS time series; neo-tectonics; Antarctica geodynamics; crustal deformations; PNRA 1. Introduction In geosciences, GNSS (Global Navigation Satellite Systems) and gravimetric measurements are key methods to highlight geodynamic and geophysical phenomena of a geological region, and are a fundamental support in the formulation of deformation models at regional and continental scale, referring also to the interaction mechanisms of interaction between the solid Earth and the cryosphere. The joint analysis of residual geodetic and gravimetric signatures in Antarctica could support theories on the structure and the kinematic of the Earth’s lithosphere, characterizing the crustal thickness and the tectonic processes. Furthermore, it could provide evidence of active geodynamic and deformation processes that change the shape of the continents. However, the observed solid Earth deformation includes several signals produced by different geophysical phenomena, which are not easy to separate, being related to phenomena that act in a very different spatial and temporal scale or due to bias in modeling or limitations in the a priori hypothesis. Difficulties in the analysis of gravimetric and geodetic data could also arise from gaps and inhomogeneity in the spatial distribution and measurement accuracy (e.g., gravimetric maps based on data collected after terrestrial and airborne gravimetric campaigns across the last two decades). Many continuous GNSS stations were installed in the last two decades in Antarctica by national and international projects for different purposes. Some of them were included in the IGS (International GNSS Service) network and used in the ITRF (International Terrestrial Reference Frame) computation. Several authors processed subsets of such stations within studies related to geodynamics, glaciology and Glacial Isostatic Adjustment (GIA) modeling [1–6], but the data processing at continental scale of the full GNSS dataset has not been performed yet. The drawback of using permanent GNSS stations in the detection of surface motions could be attributed to the limited number and density of stations whenever a detailed picture of regional deformation phenomena is sought to support geodynamic modeling. A couple of projects oriented to the detailed investigation of the regional deformation field by dense GNSS measurements were implemented by the United States and Italian projects which are, respectively, called TAMDEF (Trans Antarctic Mountains Deformation) and VLNDEF (Victoria Land Network for Deformation control) [7–10]. The mapping of Earth’s gravity field and crustal thickness in Antarctica has been improved by the CHAMP (Challenging Minisatellite Payload) [11], GRACE (Gravity Recovery and Climate Experiment) and GOCE (Gravity field and steady state Ocean Circulation Explorer) satellite gravity missions [12]. Such dedicated gravity satellite missions provided data with a full coverage and consistent accuracy at spatial resolution of the order of 100 km [12–14], but a “polar gap” South of −83.3◦ latitude still exists due to satellite orbit inclination [12]. Towards the definition of a refined gravity field over the whole Antarctic continent, the recent use of Bedmap2 [15] to better define the gravity effects from topographic mass modeling must be mentioned [12]. Over the last two decades, airborne gravimetric campaigns have improved the definition of the gravity field in confined regions (see papers [16,17], among others). In order to obtain a better spatial resolution in the gravity field mapping with delineation of crustal features, terrestrial gravity compilation should be used [17]. Unfortunately, due to logistic difficulties in terrestrial gravimetric surveys, the Antarctic territories suffer from a lack of gravity data distribution and improvement in airborne gravimetric surveys does not allow the full spatial coverage of gravimetric measurements. A first compilation of continental scale gravity anomaly grid for Antarctica was provided by Scheinert [18], on the initiative of the International Association of Geodesy (IAG) Sub-Commission 2.4f “Gravity and Geoid in Antarctica” (AntGG), as a collection and integration of 13 million gravity data points from terrestrial, airborne and shipborne surveys acquired by the international geosciences community [18]. The authors carefully considered a large Remote Sens. 2018, 10, 1608 3 of 27 Remote Sens. 2018, 10, x FOR PEER REVIEW 3 of 27 heterogeneity within the gravity dataset during the processing and provided the grid of Free-Air processing and provided the grid of Free-Air Gravity (FAG) and Bouguer Gravity (BG) anomalies Gravity (FAG) and Bouguer Gravity (BG) anomalies with a grid spacing of 10 km. By using this with a grid spacing of 10 km. By using this dataset, shorter wavelength features (for instance, dataset, shorter wavelength features (for instance, intra-crustal density variations) could be more intra-crustal density variations) could be more easily resolved. easily resolved. Several international and national projects in Antarctica have contributed to model geodynamic Several international and national projects in Antarctica have contributed to model geodynamic scenarios from the cross interpretation of geodetic and geophysical data at regional and continental scenarios from the cross interpretation of geodetic and geophysical data at regional and continental scale [19,20]. These activities are coordinated by the SCAR (Scientific Committee on Antarctic scale [19,20]. These activities are coordinated by the
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